Preheat oven to 350°F. Toss garlic cloves and olive oil in small baking dish; cover. Bake until garlic begins to brown, about 35 minutes. Drain olive oil into processor. Cool 15 minutes. Peel garlic; place in processor. Add prepared horseradish and coarse salt. Puree until almost smooth.
Place rack on large rimmed baking sheet. Sprinkle beef with salt and pepper. Spread thin layer of garlic mixture on underside of beef. Place beef, garlic mixture side down, onto rack. Spread beef with remaining garlic mixture. Cover and refrigerate at least 3 hours or up to 1 day.
Position rack in bottom third of oven; preheat to 350°F. Uncover beef. Roast until thermometer inserted into top center registers 125°F for rare, about 1 hour 45 minutes. Transfer beef to platter; let stand 30 minutes. Scrape pan juices into small saucepan.
Slice beef crosswise. Rewarm juices; drizzle over beef. Serve with sour cream and horseradish sauce.